I really dislike that line. It is very popular and appears insightful, but Pratchett is completely wrong about sheep. He is taking his views on sheep from the UK (which is where he lives) where sheep are left on their own for most of the time as their is plentiful grazing and are occasionally rounded up and driven by the farmer. Middle-eastern shepherding where there is much less available grass the sheep are led by the shepherd from place to place. This is the kind that is referenced in the Bible. There has been enough research on sheep psychology to prove that they aren’t as stupid as people like to think, they are capable of pattern recognition and problem solving. Two flocks can meet and mingle, and then be separated just by them recognising which shepherd is theirs.